Remove -detachdb and stop's detach argument.

As the only BDB database left is the wallet, and it is always
detached.

Also remove IsChainFile() predicate and related chainfile-specific
logic.

src/qt/optionsdialog.cpp

@@ -128,7 +128,6 @@ void OptionsDialog::setMapper()
     /* Main */
     mapper->addMapping(ui->transactionFee, OptionsModel::Fee);
     mapper->addMapping(ui->bitcoinAtStartup, OptionsModel::StartAtStartup);
-    mapper->addMapping(ui->detachDatabases, OptionsModel::DetachDatabases);
 
     /* Network */
     mapper->addMapping(ui->mapPortUpnp, OptionsModel::MapPortUPnP);